Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
Dim strVal As String
Dim i As Long
Dim lCount As Long
Dim Ar As Variant
On Error Resume Next
Dim lType As Long
If Target.Count > 1 Then GoTo exitHandler

lType = Target.Validation.Type
If lType = 3 Then
    Application.EnableEvents = False
    newVal = Target.Value
    Application.Undo
    oldVal = Target.Value
    Target.Value = newVal
    If Target.Column = 7 Then
        If oldVal = "" Then
            'do nothing
        Else
            If newVal = "" Then
                'do nothing
            Else
                On Error Resume Next
                Ar = Split(oldVal, ", ")
                strVal = ""
                For i = LBound(Ar) To UBound(Ar)
                    Debug.Print strVal
                    Debug.Print CStr(Ar(i))
                    If newVal = CStr(Ar(i)) Then
                        'do not include this item
                        strVal = strVal
                        lCount = 1
                    Else
                        strVal = strVal & CStr(Ar(i)) & ", "
                    End If
                Next i
                If lCount > 0 Then
                    Target.Value = Left(strVal, Len(strVal) - 2)
                Else
                    Target.Value = strVal & newVal
                End If
            End If
        End If
    End If
End If

exitHandler:
  Application.EnableEvents = True
End Sub